The free-radical polymerization of styrene has been studied in the homogeneous phase of supercritical (sc) CO2 at 80 degrees C and pressures between 200 and 1500 bar. 2,2'-Azobisisobutyronitrile is used as initiator and CBr4 as chain-transfer agent. The polymerization is monitored by means of online FT-IR/NIR spectroscopy. In the presence of CO2 a solution polymerization may be carried out up to a considerable degree of monomer conversion. At 500 bar, for example, maximum styrene conversions of 34.4 and 11.9% may be reached in homogeneous phase at CO2 contents of 16.8 and 44.5 wt.-%, respectively. Analysis of the measured conversion-time profiles yields termination rate coefficients, k(t), which are by one order of magnitude larger than k(t) for styrene bulk polymerizations at identical temperature and pressure. The enhanced termination rate in fluid CO2 is assigned to the poor solvent quality of scCO(2) for polystyrene.